I'm trying to boot the Gentoo 2004.1 on a Sunblade 100. The Live-CD
boots well in text mode, if I disable framebuffer usage in linux boot
prompt.

If I try to start the live-cd just with "gentoo-2.4" (I want to use the
framebuffer), the monitor just remains as it is. It doesn't switch to
framebuffer mode. I can also see no boot messages, but the system
boots in the background.

What I tried so far is the following:

at OPB prompt:
ok setenv output-device screen:r1024x768x70

at Linux boot prompt
gentoo-2.4 video=atyfb:1024x768@70

I tried the same thing for the equal resolution, but for 60Hz refresh
rate, but no success so far.

How can I gentoo make working with my "SUN Expert3D-Lite"
Video Adapter (ATI Chipset, I guess)??

The Expert3D-Lite card is unfortunately unsupported under Linux, so you won't be able to get anything other than perhaps a prom console on it. Supposedly these cards are based on the ATI Radeon chipset but no-one with enough desire, knowledge and/or time has one and looked into writing a driver IIRC.